Various polysaccharides such as cellulose, starch, and guar gum have been used as binders of components in solid and aqueous media. Such binders can be selected for their biodegradable properties, which can be beneficial for uses such as hydroseeding and other agricultural or erosion control applications.
Hydroseeding involves the binding of seeds to materials such as fertilizer, mulch, and other ingredients that can be incorporated into a desired “microclimate” for the improved spreading and germination of the seeds. Polysaccharide compositions such as slurries containing fibrous additives, with or without seeds, may also be sprayed onto mulch, wood chips, hay, straw, or other plant fibers, to hold these substrates into place and thereby assist in reducing erosion over a ground area. In another exemplary application, biodegradable landfill covers may be formed by spraying garbage piles with polysaccharide binder compositions.
The effectiveness of polysaccharide-based binders in these and other applications, however, can be limited due to their high water solubility and reduced mechanical strength upon contact with moisture (e.g., from rain or moist soil). For this reason, attempts have been made to cross link polysaccharides to render them less soluble or insoluble and thereby slow their degradation rate, without also preventing their ultimate conversion into harmless byproducts, namely carbon dioxide and water.
For example, metal-based cross linking agents for promoting the gellation of natural polysaccharide gums in aqueous systems have been used in the oil well drilling for the rheology control of oil well drilling fluids. These agents are described in U.S. Pat. No. 3,644,171 (the disclosure of which is incorporated by reference in its entirety) but rely, however, on the heavy metals antimony and chromium, which are undesirable from an environmental standpoint. U.S. Pat. No. 5,459,181 (the disclosure of which is incorporated by reference in its entirety) describes the use of other catalyst systems (e.g., ammonium sulfate) to catalyze the cross linking of a water soluble polysaccharide such as guar gum or hydroxyethyl cellulose with an amine formaldehyde condensate prepolymer (e.g., dimethylol urea). Other known polysaccharide cross linking agents include metallic catalysts such as ammonium zirconium carbonate which, again, lead to the deposition of unwanted metals in the soil and other areas to which they become exposed.
There remains a need for biodegradable binders for solid components, and in particular polysaccharide binders that are cross linked to decrease water solubility. Ideally, these binders should not rely on heavy metal catalysts or other cross linking agents that lead to the release of undesirable contaminants such as antimony, chromium, zirconium, and/or formaldehyde into the environment. Such binders would be applicable for uses described above (e.g., hydroseeding) as well as in other areas where cross linked polysaccharides having improved strength and a reduced tendency to solubilize or hydrolyze in aqueous media are desirable. Other types of solid components that might be advantageously bound with polysaccharide binders include mill scale particles and even dust particles and other fine solid materials in the mitigation of airborne particulates.
Industrially, processes for the purification of liquid suspensions or dispersions (and especially aqueous suspensions or dispersions) to remove suspended solid particles have been prevalent. Froth flotation, for example, is a separation process based on differences in the tendency of various materials to associate with rising air bubbles. Additives are often incorporated into the froth flotation liquid (e.g., aqueous brine) to improve the selectivity of the process. For example, “collectors” can be used to chemically and/or physically absorb onto mineral(s) (e.g., those comprising value metals) to be floated, rendering them more hydrophobic. On the other hand, “depressants,” typically used in conjunction with collectors, render other materials (e.g., gangue minerals) less likely to associate with the air bubbles, and therefore less likely to be carried into the froth concentrate.
In this manner, some materials (e.g., value minerals or metals) will, relative to others (e.g., gangue materials), exhibit preferential affinity for air bubbles, causing them to rise to the surface of the aqueous slurry, where they can be collected in a froth concentrate. A degree of separation is thereby affected. In less common, so-called reverse froth flotations, it is the gangue that is preferentially floated and concentrated at the surface, with the desired materials removed in the bottoms. Gangue materials typically refer to quartz, sand and clay silicates, and calcite, although other minerals (e.g., fluorite, barite, etc.,) may be included. In some cases, the material to be purified comprises predominantly such materials, and the smaller amounts of contaminants are preferentially floated. For example, in the beneficiation of kaolin clay, a material having a number of industrially significant applications, iron and titanium oxides can be separated by flotation from the impure, clay-containing ore, leaving a purified kaolin clay bottoms product.
The manner in which known collectors and depressants achieve their effects is not understood with complete certainty, and several theories have been proposed to date. Depressants, for example may prevent the gangue minerals from adhering to the value materials to be separated, or they may even prevent the collector(s) from absorbing onto the gangue minerals. Whatever the mechanism, the ability of a depressant to improve the selectivity in a froth flotation process can very favorably impact its economics.
Overall, froth flotation is practiced in the beneficiation of a wide variety of value materials (e.g., mineral and metal ores and even high molecular weight hydrocarbons such as bitumen), in order to separate them from unwanted contaminants that are unavoidably co-extracted from natural deposits. In the case of solid ore beneficiation, the use of froth flotation generally comprises grinding the crude ore into sufficiently small, discrete particles of a value mineral or metal and then contacting an aqueous “pulp” of this ground ore with rising air bubbles, typically while agitating the pulp. Prior to froth flotation, the crude ore may be subjected to any number of preconditioning steps, including selective crushing, screening, desliming, gravity concentration, electrical separation, low temperature roasting, and magnetic differentiation.
Another particular froth flotation process of commercial significance involves the separation of bitumen from sand and/or clay, which are ubiquitous in oil sand deposits, such as those found in the vast Athabasca region of Alberta, Canada. Bitumen is recognized as a valuable source of “semi-solid” petroleum or heavy hydrocarbon-containing crude oil, which can be upgraded into many valuable end products including transportation fuels such as gasoline or even petrochemicals. Alberta's oil sand deposits are estimated to contain 1.7 trillion barrels of bitumen-containing crude oil, exceeding the reserves in all of Saudi Arabia. For this reason, significant effort has been recently expended in developing economically feasible operations for bitumen recovery, predominantly based on subjecting an aqueous slurry of extracted oil sand to froth flotation. For example, the “Clark Process” involves recovering the bitumen in a froth concentrate while depressing the sand and other solid impurities.
Various gangue depressants for improving froth flotation separations are known in the art and include sodium silicate, starch, tannins, dextrins, lignosulphonic acids, carboxylmethyl cellulose, cyanide salts and many others. More recently certain synthetic polymers have been found advantageous in particular beneficiation processes involving froth flotation. Overall, despite the large offering of flotation depressants and dewatering agents in the art, an adequate degree of refinement in many cases remains difficult to achieve, even, in the case of froth flotation, when two or more sequential “rougher” and “cleaner” flotations are employed. There is therefore a need in the art for agents that can be effectively employed in a wide range of separation processes, including both froth flotation and the separation of solid contaminants from liquid suspensions.
Other processes, in addition to froth flotation, for the separation of solid contaminants from liquid suspensions can involve the use of additives that either destabilize these suspensions or otherwise bind the contaminants into larger agglomerates. Coagulation, for example, refers to the destabilization of suspended solid particles by neutralizing the electric charge that separates them. Flocculation refers to the bridging or agglomeration of solid particles together into clumps or flocs, thereby facilitating their separation by settling or flotation, depending on the density of the flocs relative to the liquid. Otherwise, filtration may be employed as a means to separate the larger flocs.
The additives described above, and especially flocculants, are often employed, for example, in the separation of solid particles of rock or drill cuttings from oil and gas well drilling fluids. These drilling fluids (often referred to as “drilling muds”) are important in the drilling process for several reasons, including cooling and lubricating the drill bit, establishing a fluid counterpressure to prevent high-pressure oil, gas, and/or water formation fluids from entering the well prematurely, and hindering the collapse of the uncased wellbore. Drilling muds, whether water- or oil-based, also remove drill cuttings from the drilling area and transport them to the surface. Flocculants such as acrylic polymers are commonly used to agglomerate these cuttings at the surface of the circulating drilling mud, where they can be separated from the drilling mud.
Other uses for flocculants in solid/liquid separations include the agglomeration of clays that are suspended in the large waste slurry effluents from phosphate production facilities. Flocculants such as anionic natural or synthetic polymers, which may be combined with a fibrous material such as recycled newspaper, are often used for this purpose. The aqueous clay slurries formed in phosphate purification plants typically have a flow rate of over 100,000 gallons per minute and generally contain less than 5% solids by weight. The dewatering (or settling) of this waste clay, which allows for recycle of the water, presents one of the most difficult problems associated with reclamation. The settling ponds used for this dewatering normally make up about half of the mined area, and dewatering time can be on the order of several months to several years.
In the separation of solids from aqueous liquids, other specific applications of industrial importance include the filtration of coal from water-containing slurries (i.e., coal slurry dewatering), the processing of sewage to remove contaminants (e.g., sludge) via sedimentation, and the processing of pulp and paper mill effluents to remove suspended cellulosic solids. The dewatering of coal poses a significant problem industrially, as the BTU value of coal decreases with increasing water content. Raw sewage, both industrial and municipal, requires enormous processing capacity, as wastes generated by the U.S. population, for example, are collected into sewer systems and carried along by approximately 14 billion gallons of water per day. Paper industry effluent streams likewise represent large volumes of solid-containing aqueous liquids, as waste water generated from a typical paper plant often exceeds 25 million gallons per day. The removal of sand from aqueous bitumen-containing slurries generated in the extraction and subsequent processing of oil sands, as described previously, poses another commercially significant challenge in the purification of aqueous liquid suspensions. Also, the removal of suspended solid particulates is often an important consideration in the purification of water, such as in the preparation of drinking (i.e., potable) water. Synthetic polyacrylamides, as well as naturally-occurring hydrocolloidal polysaccharides such as alginates (copolymers of D-mannuronic and L-guluronic acids) and guar gum are conventional flocculants in this service.
The above applications therefore provide several specific examples relating to the purification of aqueous liquid suspensions to remove solid particulates. However, such separations are used in a number of other processes in the mineral, chemical, industrial and municipal waste; sewage processing; and paper industries, as well as in a wide variety of other water-consuming industries. Thus, there is a need in the art for additives that can effectively promote selective separation of a wide variety of solid contaminants from liquid media. Advantageously, such agents should be selective in chemically interacting with the solid contaminants, through coagulation, flocculation, or other mechanisms such that the removal of these contaminants is easily achieved. Especially desirable are additives that are also able to complex unwanted ionic species such as metal cations to facilitate their removal as well.